SUMMARY / ABSTRACT Heavy-ion radiation therapy (HIRT) differs from other radiotherapy modalities such as x rays and protons as these high-LET (Linear Energy Transfer) radiations deposit energy far more densely on a microscopic scale. There is currently strong interest in the introduction of HIRT to the U.S., largely based on the experience of carbon-ion radiotherapy in Japan and Germany, where very encouraging survival rates have been reported for a number of hard-to-treat cancers such as pancreas, rectum and sarcomas. For example, 2-year survival of 50 to 65% has been reported after combined carbon-ion and gemcitabine chemotherapy for locally-advanced pancreatic cancer, remarkably encouraging at a post-treatment time when survival is dominated by distant metastases. Thus there has been much discussion that, as well as producing local effects to the tumor, HIRT may also be inducing long-range systemic anti-cancer effects. However, the underlying mechanisms for such high-LET-induced long-range systemic effects are not understood and there is evidence that the classic radiobiological phenomena underlying the efficacy of conventional x-ray radiotherapy, while still potentially relevant for local tumor control, are not the dominant phenomena driving the potential systemic efficacy of HIRT. Rather the data suggest different high-LET-induced mechanisms underlying radiation-induced long- range anti-cancer effects ? and what is not known is the LET dependence of these long-range effects. In this BRP, and leveraging from the unique technologies and skillsets at the Radiological Research Accelerator Facility (RARAF) and the Laboratory for Functional Optical Imaging (LFOI), novel tools will be developed to study and understand long-range radiation-induced biological effects, and particularly their dependence on LET. The key tools will be 1) a series of mono-LET ion beams providing spatially defined 3-D exposures, integrated with 2) SCAPE (Swept Confocally-Aligned Planar Excitation) wide-area 3D microscopy, imaging within and outside the radiation field. In parallel, the BRP tools will be applied to address the central hypothesis of LET dependence of long-range radiation effects. These studies will encompass increasing levels of complexity from tumor cells through in-vitro tumor/tissue models to in-vivo tumor models. To develop and apply these technologies, an interdisciplinary team has been assembled of accelerator physicists and radiobiologists from RARAF, and biomedical engineers from LFOI, enhanced through continuous engagement with internationally recognized scientists and clinicians with experience in HIRT. Apart from the primary goal of optimizing HIRT efficacy, understanding the relevant LET dependencies in HIRT will provide a pathway for determining the optimal ion / ions for its use ? a key outcome that in turn will likely determine the future worldwide usage of HIRT, in that the capital cost of HIRT is dominated by the choice of ion or ions to be used. If, for example, the optimal LET range for HIRT could be achieved with helium ions, a helium therapy machine would be far smaller and cheaper than a >$150M carbon-ion machine.